(1) Field of the Invention
The invention relates to a photodetector, more particularly, to semiconductor photodetector devices serving as image elements of a solid state imaging device.
(2) Description of the Relate Art
Various devices, such as a bipolar transistor type device, an insulating gate field effect transistor (FET), and a charge coupled device (CCD) are used as the semiconductor photodetector devices in the solid state imaging device. All of the above-mentioned devices have a P-type (or N-type) silicon substrate and an N-type (or P-type) region therein so as to form a PN junction, and utilize the photovoltaic-effect brought about by illuminating the PN junction region with light. Such a light receiving structure basically corresponds to a P-N junction photodiode. In this case, when light falls on the P-N junction photodiode in which a P-type silicon substrate is grounded and a N-type region formed in the substrate is at a positive potential, i.e., the PN junction is reverse-biased, the energy of the light photons creates electron-hole pairs in a depletion region by exciting electrons in the P-type substrate, and causing holes to form in the N-type region due to photoexcitation. The electron-hole pairs are separated by the electric field, leading to a current flow in an external circuit as carriers drift across the depletion region. The total current (photocurrent) of the P-N photodiode is the drift current due to carriers generated in the depletion region and the diffusion current due to carriers generated in the P-type substrate and the N-type region and diffusing into the PN junction.
Such a PN junction photodiode brings about the following disadvantages:
(i) Since an electric charge generated by light irradiation is allowed to flow, the sensitivity of the photodiode to a faint light is low.
(ii) Where plural PN junction photodiodes are closely arranged in a solid state imaging device, and when a very strong light falls on one of those photodiodes, a portion of the electric charge generated by light irradiation flows into an adjacent photodiode so that the resolution of the imaging device is deteriorated.
(iii) Since electrons generated in the P-type substrate by a strong light do not easily disappear, such electrons bring about a photo-current flow continuing for a while after the extinguishment of the light, i.e., so-called lag or persistence.